This invention relates to an apparatus for diagnosing performances of an internal combustion engine ignition system and more particularly to measurement of stray capacitance having great influence on transmission of high voltage and secondary generation voltage representative of spark capability.
In present-day internal combustion engine ignition systems, high voltage generated by ignition coil is transmitted to a spark plug through a high tension cord and a distributor. Since the ignition coil has a relatively high output impedance and the high tension cord runs above an engine body in close proximity thereof, a distributed capacitance called stray capacitance always exists at a secondary distribution portion of the ignition coil. This stray capacitance increases when the high tension cord is deposited with, for example, water or salt water droplets and as a result, high voltage to be applied to spark plug electrodes is decreased as compared with voltage originally generated at the ignition coil. FIG. 1 shows such a state, where abscissa represents a stray capacitance C and ordinate represents a maximum value E of generated voltage, and wherein curves a and b are representative of characteristics for ignition coil primary breaking current values of 5.7 amperes and 3.8 amperes, respectively. While the voltage generated at the ignition coil easily decreases with increase in the stray capacitance in this manner, such an exhaust gas countermeasure as exhaust gas recirculation (EGR) requires a more increased high voltage of the voltage generated at the ignition coil. Accordingly, the probability that the mis-spark occurs becomes high and this imposes a problem on engine performances.
To cope with this problem, it is necessary to develop an ignition coil and a high tension cord which are of high reliability and resistant to voltage reduction. In implementing these countermeasures, an ignition system diagnostic apparatus, especially, a stray capacitance measuring apparatus and a secondary generation voltage measuring apparatus are required.
Incidentally, a commercial electrostatic capacitance meter may possibly be available for measurement of the stray capacitance. However, this meter has great difficulties with the measurement in that the ignition coil is normally separated from the spark plug by the distributor and in that a high voltage is applied. In addition, it is almost impossible to record actual running status with this meter. For measurement of the secondary generation voltage, it is a general practice to measure a maximum value of an open waveform obtained under the condition that the secondary side of the ignition system is isolated from earth to prevent from occurring a break to make discharge. But, during actual running of vehicles, discharge is carried out continuously so that measurement of the secondary generation voltage by this method is impossible.